Method For Applying A Protective Layer, Component Coated With A Protective Layer, And Gas Turbine Comprising Such A Component

ABSTRACT

A method for applying a protective layer resistant to high-temperature degradation by corrosion and erosion to a base metal, comprises: applying a MCrAlY-based bond layer to the base metal; coating, by overaluminizing with an Al diffusion layer, the bond layer; subjecting the Al diffusion layer ( 14 ) to abrasion treatment so that an outer build-up layer is removed from the Al diffusion layer; and applying a ceramic thermal barrier coating of yttria partially stabilized zirconia to the remaining Al diffusion layer. The ceramic thermal barrier coating is applied to the remaining Al diffusion layer by air plasma spraying. The applied bond layer is subjected to a polishing treatment before being overaluminized such that a surface roughness of Ra≦2 μm is produced at the bond layer.

PRIORITY CLAIM

This is a U.S. national stage of application No. PCT/EP2012/060195, filed on May 31, 2012. Priority is claimed on the following application: Country: Germany, Application No.: 10 2011 103 731.8, Filed: May 31, 2012, the content of which is incorporated in its entirety herein by reference.

BACKGROUND OF THE INVENTION

The invention is directed to a method for applying a protective layer to a base metal and to a component part coated with a protective layer for use in a hot gas region of a gas turbine, and to a gas turbine with a component part of this kind.

A method of the type mentioned above is known, e.g., from EP 1 637 622 A1.

In modern gas turbines, the surfaces in the hot gas region are provided almost entirely with coatings. The blades of rear turbine rows may be excepted in some cases. The thermal barrier coatings (TBCs) used for this purpose serve to lower the material temperature of cooled component parts. In this way, the life of the component parts can be prolonged, cooling air can be saved, or the gas turbines can be operated at higher input temperatures.

Thermal barrier coating systems are always formed of a metallic bond layer which is diffusion bonded to the base material (base metal) and a ceramic layer on top of the bond layer. This ceramic layer has poor thermal conductivity and provides the actual barrier against heat flow and protects the base metal against high-temperature degradation by corrosion and erosion. The generally accepted ceramic material for the thermal barrier coating is zirconium oxide partially stabilized with approximately 7 weight percent of yttrium oxide (the international acronym for yttria partially stabilized zirconia is YPSZ).

Thermal barrier coatings are divided into two different categories depending on the method of application.

A first category consists of thermal barrier coatings that are vapor deposited by electron beam physical vapor deposition methods (EB-PVD methods). When certain deposition conditions are maintained, these thermal barrier coatings have a columnar, strain-tolerant structure and therefore provide a particularly good resistance to thermal cycle fatigue (TCF). In the associated methods for applying the thermal barrier coating, the thermal barrier coating is chemically bonded to a layer of pure alumina (Thermally Grown Oxide, TGO), which is formed by the bond layer during application thereof and subsequently during service through the formation of an alumina-zirconia mixture. On the one hand, this method imposes particular demands for the oxide growth on the bond layer, but on the other hand it ensures an especially strong bond.

A second category includes thermal barrier coatings that are sprayed on thermally (usually by air plasma spraying [APS]). These thermal barrier coatings have a porosity of between approximately 10 and 25 volume percent depending on the desired layer thickness and stress distribution. Due to the fact that the ceramic layer is bonded to the bond layer mechanically in this case, the bond layer is deliberately in rough condition when sprayed in order to maximize the interface and, therefore, the adhesive forces. A certain chemical bonding is brought about through TGO formation only after a long period of service. This application method is relatively simple resulting in relatively favorable coating costs.

SUMMARY OF THE INVENTION

It is the object of the invention to further develop a method in such a way that a good resistance to thermal fatigue is achieved in the protective layer while still allowing the method to be carried out in a simple manner.

A further object of the invention is to provide a component part for use in a hot gas region of a gas turbine, which component part is coated with a protective layer that is resistant to high-temperature degradation by corrosion and erosion, and to provide a gas turbine having a component part of this type, wherein the protective layer can be produced on the component part in a simple manner and has a good resistance to thermal fatigue.

With regard to the bond layer—preferably in stationary gas turbines—thermally sprayed MCrAlY-based (M=Ni, Co) undercoats are used. MCrAlY layers contain the intermetallic β phase NiCoAl as aluminum reserve in a NiCoCr (“Y”) matrix. But this also has an embrittling effect so that the Al content that can be realized in the MCrAlY layer in practice is ≦12 weight percent.

To further enhance resistance to oxidation, the MCrAlY coats are overaluminized with an Al diffusion layer. Owing to the risk of embrittlement, this is largely limited to low-aluminum (Al≦8%) starting layers.

The structure of an overaluminized MCrAlY layer includes an inner, substantially unchanged mixed Y, β phase, i.e., a diffusion zone, in which the Al content increases to about 20%, and an outer β-NiAl phase with an Al proportion of about 30%. This outer β-NiAl phase represents a certain weak point in the layer system as regards brittleness and sensitivity to cracking. Therefore, the overaluminized coat is subjected to an abrasion treatment so that the outer β-NiAl phase is removed down to the diffusion zone. This also has a favorable effect on the aluminum activity and thus benefits the capability of TGO formation.

In this regard, a good bonding of the ceramic layer can be achieved without the need for a rough bond layer so that it is possible inter alia to apply the MCrAlY layer by means of low pressure plasma spraying (LPPS) or thermal spraying, for example, high velocity flame spraying (HVOF) or vacuum plasma spraying. High velocity flame spraying is more economical and tends to generate smoother surfaces.

According to a first aspect of the invention, a method is provided for applying a protective layer that is resistant to high-temperature degradation by corrosion and erosion to a base metal, wherein a MCrAlY-based bond layer is applied to the base metal, the bond layer is coated by overaluminizing with an Al diffusion layer, the Al diffusion layer is subjected to abrasion treatment so that an outer build-up layer is removed from the Al diffusion layer, and a ceramic thermal barrier coating of yttria partially stabilized zirconia is applied to the remaining Al diffusion layer. The method according to the invention is characterized in that the ceramic thermal barrier coating is applied to the remaining Al diffusion layer by air plasma spraying.

In another aspect, the applied bond layer is subjected to a polishing treatment before being soveraluminized. A surface roughness of Ra≦2 μm is preferably produced at the bond layer by the polishing treatment.

In another aspect, the bond layer is applied to the base metal by thermal spraying, for example, high velocity flame spraying (HVOF) or vacuum plasma spraying high velocity flame spraying or deposition from the vapor phase.

According to a further aspect, the Al diffusion layer is subjected to a polishing treatment after the abrasion treatment such that a surface roughness of Ra≦2 μm is produced at the remaining Al diffusion layer.

According to yet another aspect, a heat treatment is carried out after overaluminizing the bond layer and before the abrasion treatment of the Al diffusion layer in order to influence the mechanical properties of the base metal.

According to yet another aspect, during overaluminizing an inner diffusion zone with an Al content of about 20 weight percent is produced in the Al diffusion layer and the outer build-up layer with an Al content of about 30 weight percent is produced on the diffusion zone, and the outer build-up layer of the Al diffusion layer is removed by abrasion treatment until the Al content in a surface of the remaining Al diffusion layer amounts to more than 18 weight percent and less than 30 weight percent.

According to a second aspect of the invention, a component part is provided for use in a hot gas region of a gas turbine, wherein the component part has a surface that is at least partially provided with a protective layer which is resistant to high-temperature degradation by corrosion and erosion and which is applied by a method according to one or more or all of the above-described embodiment forms of the invention in any conceivable combination.

According to a third aspect of the invention, there is provided a gas turbine with a hot gas region and with a component part according to the second aspect of the invention arranged therein.

Owing to the application of the method according to the invention for producing the protective layer on the component part, the protective layer has a good resistance to thermal fatigue and can nevertheless be produced in a simple manner.

Finally, the invention provides a thermal barrier coating concept which combines the favorable costs of the APS method with the advantages of chemical bonding between the bond layer and the ceramic layer. In this way, TCF behavior can be improved over that of conventional APS layers. Accordingly, thermal barrier coatings with improved resistance to thermal fatigue can be produced in a simpler manner and, therefore, at a lower cost than with EB-PVD methods.

The invention expressly extends to embodiment forms which are not given by combinations of features from explicit references of the claims so that the disclosed features of the invention can be combined with one another in any way insofar as technically meaningful.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described with reference to a preferred embodiment form and with reference to the accompanying drawing, in which:

FIG. 1 is a sectional view of a region of a component part of a gas turbine according to an exemplary embodiment of the invention, which component part is arranged in a hot gas region and is provided with a protective layer.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 is a sectional view showing a region of a component part 10 of a gas turbine 1 according to an exemplary embodiment of the invention, which component part 10 is arranged in a hot gas region and is provided with a protective layer 12-14.

For protection against high-temperature degradation by corrosion and erosion, the component part 10, e.g., a turbine blade or other component part of the gas turbine 1 coming into contact with hot gas, has a base metal 11 (base material) with a surface provided in its entirety or partially with a ceramic thermal barrier coating that is resistant to high-temperature corrosion and erosion. The ceramic thermal barrier coating 13 is formed of zirconium oxide that is partially stabilized with approximately 7 weight percent of yttrium oxide (the international acronym for yttria partially stabilized zirconia is YPSZ).

To improve the adhesion of the thermal barrier coating 13 to the base metal 11, an undercoat or bond layer 12 is first applied to this base metal 11 (to the surface thereof). The bond layer 12 comprises a special alloy based on MCrAlY (e.g., LCO 22). Here the letter M represents Ni or Co or a combination thereof. The bond layer 12 is applied by means of low pressure plasma spraying (LPPS) or, as is preferable in this case, by high velocity flame spraying (HVOF).

The applied bond layer 12 is subsequently subjected to a polishing treatment (e.g., fine finishing) by which a surface roughness of Ra≦2 μm is generated on the bond layer 12.

To increase the Al content in the bond layer 12, this bond layer 12 is then coated by overaluminizing with an Al diffusion layer 14. The overaluminizing can be realized by means of a treatment in which a reactive Al-containing gas, possibly an Al halide (AlX₂), causes an inward diffusion of Al combined with an outward diffusion of Ni, e.g., a chemical vapor deposition (CVD), at high temperatures.

The overaluminizing causes an inner diffusion zone 14.1 with an Al content of about 20 weight percent to be formed in the Al diffusion layer 14 on the largely unchanged bond layer 12 and thereon an outer build-up layer 14.2 comprising a brittle β-NiAl phase with an Al content of about 30 weight percent.

After overaluminizing the bond layer 12, heat treatment may be performed to influence or adjust the mechanical properties of the base metal 11.

Subsequently, the outer build-up layer 14.2 is removed down to the inner diffusion zone 14.1 of the Al diffusion layer 14 by abrasion treatment, e.g., blasting with hard particles (e.g., corundum, silicon carbide, cut metal wire, etc.) or treating with other known abrasive media or polishing media. The abrasion treatment is carried out until the surface of the remaining Al diffusion layer 14 (diffusion zone 14.1) has an Al content of greater than approximately 18 weight percent and less than approximately 30 weight percent.

After abrading, the Al diffusion layer 14 is subjected to a polishing treatment (e.g., fine finishing) such that a surface roughness of Ra≦2 μm is produced at the remaining Al diffusion layer 14 (diffusion zone 14.1).

The ceramic thermal barrier coating (YPSZ ceramic layer) 13 is then applied by air plasma spraying (APS) to the surface of the remaining AL diffusion layer 14 which has been prepared as described above. The same parameters can be used for the APS method as for conventional bond layers.

Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 

1-9. (canceled)
 10. A method for applying a protective layer resistant to high-temperature degradation by corrosion and erosion to a base metal (11), the method comprising: applying a MCrAlY-based bond layer (12) to the base metal (11); coating, by overaluminizing with an Al diffusion layer (14), the bond layer (12); subjecting the Al diffusion layer (14) to abrasion treatment so that an outer build-up layer (14.2) is removed from the Al diffusion layer (14); and applying a ceramic thermal barrier coating (13) of yttria partially stabilized zirconia to the remaining Al diffusion layer (14), wherein the ceramic thermal barrier coating (13) is applied to the remaining Al diffusion layer (14) by air plasma spraying, wherein the applied bond layer (12) is subjected to a polishing treatment before being overaluminized such that a surface roughness of Ra≦2 μm is produced at the bond layer (12).
 11. The method according to claim 10, wherein the bond layer (12) is applied to the base metal (11) by a thermal spraying method.
 12. The method according to claim 10, wherein the Al diffusion layer (14) is subjected to a polishing treatment after the abrasion treatment such that a surface roughness of Ra≦2 μm is produced at the remaining Al diffusion layer (14).
 13. The method according to claim 10, wherein a heat treatment is carried out after overaluminizing the bond layer (12) and before abrading the Al diffusion layer (14) in order to influence the mechanical properties of the base metal (11).
 14. The method according to claim 1, wherein during overaluminizing an inner diffusion zone (14.1) with an Al content of about 20 weight percent is produced in the Al diffusion layer (14) and the outer build-up layer (14.2) with an Al content of about 30 weight percent is produced on the diffusion zone (14.1), and in that the outer build-up layer (14.2) of the Al diffusion layer (14) is removed by abrasion treatment until the Al content in a surface of the remaining Al diffusion layer (14) amounts to more than 18 weight percent and less than 30 weight percent.
 15. A component part (10) for use in a hot gas region of a gas turbine (1), the component part (10) having a surface that is at least partially provided with a protective layer resistant to high-temperature degradation by corrosion and erosion and applied by the method according to claim
 10. 16. A gas turbine (1) with a hot gas region, the gas turbine having a component part (10) according to claim
 15. 